The present invention relates to rotary-wing aircraft such as helicopters. More particularly, the invention relates to a method and a system for trimming a rotary-wing aircraft about its pitch axis, i.e., for causing the rotary-wing aircraft to maintain a particular orientation about its pitch axis.
Rotary-wing aircraft such as helicopters typically comprise a fuselage, a tail boom fixedly coupled to and extending from the fuselage, a pylon fixedly coupled to an end of the tail boom, an engine and transmission mounted on the fuselage, and a rotor coupled to the engine and transmission by a rotatable mast, or drive shaft. The rotor comprises a centrally-located hub, and a plurality of rotor blades coupled to the hub and extending radially outward from the hub. The rotor blades generate lift that suspends the fuselage below the rotor during flight. The overall lift is typically controlled by a collective control that collectively varies the pitch of the rotor blades. Directional control of the helicopter is usually achieved, in part, by a cyclic control that varies the pitch of each rotor blade on a cyclic basis so as to asymmetrically vary the overall lift.
The hub of a helicopter must be tilted forward for the helicopter to fly in the forward direction. Helicopters are often constructed so that the rotor is tilted in relation to the fuselage by several degrees, i.e., the plane of rotation of the hub and the rotor blades is angled in relation to the longitudinal axis of the fuselage. This feature, under certain conditions, allows the fuselage to remain level, or nearly level, as the helicopter is flying forward. (The aerodynamic drag exerted on the fuselage is believed to be at or near its minimum when the fuselage is level in relation to the direction of flight.)
The rotor is tilted in relation to the direction of flight through the use of the cyclic control. Helicopters are typically designed so that minimal cyclic control is needed when the helicopter is operating under normal cruise conditions, and with its center of gravity well within limits. The amount of cyclic control needed to tilt the rotor generally increases, however, as the helicopter is operated at off-cruise conditions, or with a center of gravity approaching its forward or aft limits. Increasing the amount of cyclic control increases the amount of engine power, and thus the amount of fuel, needed to maintain a given flight condition. Hence, operating a helicopter with its center of gravity at or near limits typically increases the fuel consumption (or lowers the airspeed) of the helicopter. (This type of operating condition can routinely occur due to passenger or cargo loading, or as fuel is burned during flight.)
Moreover, operating a helicopter with its center of gravity at or near limits may necessitate tilting the rotor to an extreme that causes the fuselage to tilt substantially in relation to the direction of flight. Tilting the fuselage in this manner substantially increases the drag thereon, resulting in increased fuel consumption or reduced airspeed.
Many helicopters are equipped with a horizontal stabilizer coupled to the pylon or the tail boom. The horizontal stabilizer typically is a wing-like structure, and increases the longitudinal stability of the helicopter, i.e., the stability of the helicopter about its pitch axis, during forward flight. A horizontal stabilizer can be fixed in relation to the pylon or the tail boom, or movable. The position of a movable horizontal stabilizer is typically controlled in conjunction with the helicopter""s cyclic control and airspeed to augment the pitch force produced by the cyclic control.
Conventional horizontal stabilizers, in general, are not adapted to compensate for changes in the center of gravity of the helicopter, or the effect of such changes on the trim of the helicopter about its pitch axis. Compensation for changes in the center of gravity is usually achieved exclusively through the use of cyclic control. In other words, cyclic control alone is usually employed to trim the helicopter about its pitch axis in response to changes in the center of gravity (such changes commonly occur due to, for example, shifting passenger loads and fuel burn during flight).
The amount of cyclic control needed to trim a helicopter about its pitch axis can be substantial when the helicopter""s center of gravity at or near limits. In other words, compensating for a forward or aft center-of-gravity condition using the cyclic control can necessitate substantial deflection of the cyclic control from its neutral position. Operating a cyclic control in this manner is believe to increase the amount of power needed to maintain a given operating condition, and can thus lead to increased fuel consumption or lower airspeed.
Furthermore, changes in the center of gravity of a helicopter can alter the orientation of the helicopter""s fuselage in relation to the direction of flight. Conventional horizontal stabilizers, in general, are not adapted to compensate for such changes. Moreover, the cyclic control is used primarily to control the orientation of the rotor in relation to the direction of flight to attain a desired airspeed. Thus, the cyclic control is largely ineffective at controlling the orientation of the fuselage. Tilting of the fuselage in relation to the direction of flight is believed to increase the aerodynamic drag on the helicopter, and is therefore considered highly undesirable. Tilting of the fuselage and, hence, aerodynamic drag, can be substantial when the helicopter is operating with a center of gravity at or near limits.
Consequently, a need exists for a method and a system that allow a pilot to compensate for changes in the center of gravity of a helicopter without relying exclusively on the use of cyclic control.
A preferred embodiment of a rotary-wing aircraft comprises a fuselage, a tail boom fixedly coupled the fuselage, a pylon fixedly coupled to the tail boom, and a main rotor assembly rotatably coupled to the fuselage and comprising a hub and a plurality of rotor blades pivotally coupled to the hub. The preferred embodiment also comprises a cyclic control adapted to vary an orientation of the rotor blades in relation to the hub on a cyclical basis, and a horizontal stabilizer mounted on one of the pylon and the tail boom. At least a portion of the horizontal stabilizer is movable in relation to the one of the pylon and the tail boom and a position of the at least a portion of the horizontal stabilizer is controllable independent of the cyclic control.
Another preferred embodiment of a rotary-wing aircraft comprises a fuselage, a tail boom fixedly coupled to the fuselage, a pylon fixedly coupled to the tail boom, a main rotor assembly rotatably coupled to the fuselage and comprising a plurality of rotor blades, and a horizontal stabilizer mounted on one of the pylon and the tail boom. At least a portion of the horizontal stabilizer is movable in relation to the one of the pylon and the tail boom. The preferred embodiment also comprises an inclinometer mounted on the fuselage, and an actuator system adapted to vary a position of the least a portion of the horizontal stabilizer in relation to the one of the pylon and the tail boom.
Another preferred embodiment of a rotary-wing aircraft comprises a fuselage, a tail boom fixedly coupled the fuselage, a pylon fixedly coupled to the tail boom, a main rotor assembly rotatably coupled to the fuselage and comprising a hub and a plurality of rotor blades coupled to the hub, a cyclic control adapted to change an orientation of the rotor blades in relation to the hub on a cyclical basis in response to a control input, and a horizontal stabilizer mounted on one of the pylon and the tail boom. At least a portion of the horizontal stabilizer is movable in relation to the one of the pylon and tail boom. The preferred embodiment also comprises an attitude sensor mounted on the fuselage and adapted to generate an output representing an orientation of the fuselage in relation to a direction coinciding substantially a direction of level flight of the helicopter, and a sensor adapted to measure at least a portion of the control input to the cyclic control.
Another preferred embodiment of a rotary-wing aircraft comprises a fuselage, a tail boom fixedly coupled the fuselage, and a pylon fixedly coupled to the tail boom. The preferred embodiment also comprises a main rotor assembly comprising a hub, a plurality of rotor blades pivotally coupled to the hub, a drive shaft fixedly coupled to the hub and rotatably coupled to the fuselage, a first plurality of control tubes each fixedly coupled to one of the plurality of rotor blades, a first swash plate fixedly coupled to the mast and pivotally coupled to the first plurality of control tubes, a bearing, a second swash plate rotatably coupled to the first swash plate by way of the bearing, and a second plurality of control tubes pivotally coupled the second swash plate.
The preferred embodiment also comprises a cyclic control comprising a control stick and a linkage coupled to the control stick and the second plurality of control tubes. The linkage is adapted to vary a position of the second plurality of control tubes in response to movement of the control stick so that the first and second swash plate tilt and thereby vary a position of each of the first plurality of control tubes on a cyclical basis so that an orientation of each of the plurality of rotor blades varies on a cyclical basis.
The preferred embodiment further comprises a horizontal stabilizer coupled to the tail boom. At least a portion of the horizontal stabilizer is movable in relation to the one of the pylon and the tail boom independent of the cyclic control. The preferred embodiment also comprises an attitude sensor mounted on the fuselage and adapted to generate an output representing an orientation of the fuselage in relation to a direction substantially coinciding with a direction of level flight of the helicopter.
A preferred embodiment of a system comprises a horizontal stabilizer adapted to be mounted on one of a pylon and a tail boom of a helicopter. At least a portion of the horizontal stabilizer is movable in relation to the one of a pylon and a tail boom independent of a cyclic control of the helicopter. The preferred embodiment also comprises a sensor adapted to measure at least a portion of a control input to the cyclic control, an attitude sensor mounted on a fuselage of the helicopter and adapted to generate an output representing an orientation of the fuselage in relation to a direction substantially coinciding with a direction of level flight of the helicopter, and a cockpit display adapted to display the at least a portion of a control input to the cyclic control and the orientation of the fuselage in relation to a direction substantially coinciding with a direction of level flight of the helicopter.
A preferred method of operating a helicopter comprises controlling a direction of flight of the helicopter by varying a deflection of rotor blades of the helicopter on a cyclical basis using a cyclical control of the helicopter, and adjusting an angle between a longitudinal axis of a fuselage of the helicopter and a direction coinciding substantially with a direction of level flight of the helicopter by varying a position of at least a portion of a horizontal stabilizer mounted on one of a pylon and a tail boom of the helicopter independent of the cyclic control.
Another preferred method of operating a helicopter comprises controlling a pitch angle of the helicopter using a cyclic control of the helicopter, and trimming the helicopter about a pitch axis of the helicopter by varying a position of at least a portion of a horizontal stabilizer mounted on one of a pylon and a tail boom of the helicopter independent of the cyclic control.